The present invention relates to particulate mineral materials having improved properties and their use in application compositions.
Particulate mineral materials find use in a variety of applications including pigments, fillers and extenders for use in paints, plastics, polymers, paper making and paper coating.
Particulate minerals may exist in a hydrous form; Such minerals may include, for example, kaolin clay, ball clay, talc, mica, and vermiculite. Kaolin is a well-known mineral which comprises mainly the mineral kaolinite obtained by processing material obtained from natural sources. Alternatively, kaolins may be produced in a calcined or chemically aggregated form. Calcined kaolins are obtained by processing hydrous kaolins at high temperatures, e.g. greater than 800xc2x0 C. Chemically aggregated kaolins are composites having a micro structure resembling that of calcined kaolins produced by treating hydrous kaolins with chemicals. Calcined and chemically aggregated kaolins can show benefits such as improved opacity in application compositions compared with hydrous kaolins. However, such benefits are obtained only with certain disadvantages such as additional cost of production of these materials and a deleterious effect on other properties of the host composition.
The present invention is concerned with improving one or more properties of hydrous minerals. In particular, it is concerned with improving the opacity, without significantly hindering the overall combination of properties shown by such materials in host compositions.
According to the present invention in a first aspect there is provided a particulate hydrous mineral suitable for use as an opacifying pigment, filler or extender which has a shape factor which is greater than S, where S is given by Equation 1 as follows:
S=(d50/0.036)+20xe2x80x83xe2x80x83Equation 1;
where d50 is the mean particle size of the particles of the mineral measured in microns (micrometers).
The shape factor and the d50 value are measured by known procedures as described later.
We have found surprisingly and beneficially that the particulate mineral according to the invention shows an unusually high opacity compared with prior art particulate hydrous minerals and this can be obtained with acceptable or no changes in other beneficial properties in a host composition.
The particulate hydrous mineral material of the invention may be selected from the group consisting of kaolin, ball clay, talc, mica, and vermiculite. It is especially preferred that the particulate hydrous mineral is a kaolin.
The invention allows particulate hydrous mineral, and particularly hydrous kaolin, to be employed as an extender in a matt paint composition to provide an opacity which matches or approaches that which may be achieved using more expensive extenders such as calcined and chemically aggregated kaolins. Furthermore, this opacity improvement may be obtained without other disadvantages caused by use of calcined and chemically aggregated koalins.
The sheen, mud crack resistance and stain resistance of the composition are adversely affected by addition of calcined and chemically aggregated kaolins but the adverse effect on such properties can be much less using the particulate hydrous mineral, and most particularly hydrous kaolin, according to the invention. This surprising result was not predictable.
The combination of benefits shown by the particulate hydrous mineral, and most preferably the hydrous kaolin, according to the invention is not obtained with prior art hydrous minerals (and kaolins). We believe that the unusually high opacity shown by the hydrous kaolin of the invention is produced by an unusually high porosity. The mineral (particularly kaolin) porosity can be measured in a well known manner by oil absorption as described later. Without wishing to be bound by any particular theory, we believe that the high porosity results from the unusually thin kaolinite platelets which constitute the particles of the new product having properties defined by Equation 1 earlier. A collection of such platelets can exist in a form in which at least some of such platelets do not lie flat in stacks but can be arranged with their axes in a variety of different planes causing significant voids to exist between the platelets to contribute to the porosity.
The particulate mineral of the invention preferably has a fine particle size, a high shape factor, and a steep particle size distribution which are believed to enhance the opacity of the mineral in an application composition.
xe2x80x9cShape factorxe2x80x9d as used herein is a measure of an average value (on a weight average basis) of the ratio of mean particle diameter to particle thickness for a population of particles of varying size and shape as measured using the electrical conductivity method and apparatus described in GB-A-2240398/U.S. Pat. No. 5,128,606/EP-A-0528078 and using the equations derived in these patent specifications. xe2x80x9cMean particle diameterxe2x80x9d is defined as the diameter of a circle which has the same area as the largest face of the particle. In the measurement method described in EP-A-0528078 the electrical conductivity of a fully dispersed aqueous suspension of the particles under test is caused to flow through an elongated tube. Measurements of the electrical conductivity are taken between (a) a pair of electrodes separated from one another along the longitudinal axis of the tube, and (b) a pair of electrodes separated from one another across the transverse width of the tube, and using the difference between the two conductivity measurements the shape factor of the particulate material under test is determined.
The mean particle size or d50 value and other particle size properties referred to herein for the particulate mineral according to the invention are as measured in a well known manner by sedimentation of the particulate material in a fully dispersed condition in an aqueous medium using a SEDIGRAPH 5100 machine as supplied by Micromeritics Corporation. Such a machine provides measurements and a plot of the cumulative percentage by weight of particles having a size, referred to in the art as the xe2x80x98equivalent spherical diameterxe2x80x99 (esd), less than given esd values. The mean particle size d50 is the value determined in this way of the particle esd at which there are 50% by weight of the particles which have an equivalent spherical diameter less than that d50 value.
The value of d50 for the preferred particulate kaolin according to the invention may for example be in the range 0.4 xcexcm to 3 xcexcm, especially 0.5 xcexcm to 2.0 xcexcm. For example, particulate kaolin of English (Cornish) origin may have a d50 value of from 0.5 xcexcm to 1.0 xcexcm. Particulate kaolins having some other origins may have a larger d50 value.
It is not necessary for the preferred particulate kaolin of the invention to have a shape factor which is high, e.g. above 40. However, if the shape factor is less than 40 the d50 value must be less than about 0.6 xcexcm to satisfy the above relationship expressed in Equation 1. If the d50 value is greater than 1 xcexcm then the shape factor must be greater than about 50 to satisfy the above relationship of Equation 1. Similarly, if the d50 value is greater than 2 xcexcm then the shape factor must be greater than about 75. Preferably, the particulate kaolin has a shape factor of at least 40, in many cases at least 60, e.g. a shape factor in the range 70 to 140, especially from 80 to 120. In all cases, however, the shape factor is greater than S where S is defined by the relationship with d50 value given in Equation 1 above.
Preferably, the oil absorption of the preferred particulate kaolin is at least 50 grammes of linseed oil per 100 grammes of kaolin. (This may be measured in the manner described in Method A).
It is preferred that the particulate mineral, which is preferably a hydrous kaolin, according to the present invention has a shape factor which is greater than S where S is defined by Equation 2 as follows:
S=(d50/0.036)+25xe2x80x83xe2x80x83Equation 2.
The hydrous mineral according to the invention may be prepared by light comminution, e.g. grinding or milling, of a coarse mineral to give suitable delamination thereof. The comminution may be carried out by use of beads or granules of a plastics, eg. nylon, grinding or milling aid. The coarse mineral may be refined to remove impurities and improve physical properties using well known procedures. The mineral may be treated by a known particle size classification procedure, e.g. screening and/or centrifuging, to obtain particles having a desired particle size distribution and d50 value.
The steepness of the particle size distribution (xe2x80x98psdxe2x80x99) of the particulate mineral according to the invention is also important. The steepness (sometimes referred to as xe2x80x98narrownessxe2x80x99) of the psd refers to the slope of the psd curve. Thus, in some cases the psd of the mineral according to the invention may be steep and in other cases it may not steep or xe2x80x98broadxe2x80x99. The steepness can be defined in several ways. In this specification, the steepness is measured in a manner which is typical of these various ways and is expressed as 100xc3x97 the ratio of d30 to d70, where d30 is the value of the particle esd less than which there are 30% of the particles and d70 is the value of the particle esd less than which there are 70% of the particles as obtained from the psd measured as above.
The particulate mineral according to the invention, which is preferably a kaolin, may be used in the various applications in which the mineral is used as a pigment, filler or extender especially in paints, plastics, polymers and paper making and coating.
The particulate mineral according to the first aspect of the invention may be mixed with other pigments, fillers and extenders to obtain a blend of properties provided by the constituents of the mixture. The other pigment, filler or extender material may comprise one or more of hydrous kaolin, calcined kaolin, aggregated kaolin, calcium carbonate (ground or precipitated), talc, gypsum or other known white particulate mineral or pigment material. For example, it is illustrated later that the opacity of matt paint compositions containing kaolin is raised at the expense of stain resistance. However, an extender giving improved opacity but with acceptable loss of stain resistance may be obtained by use of the preferred particulate kaolin according to the invention blended with other pigment, filler or extender materials. Thus, an improvement in opacity may still be obtained by blending the particulate kaolin according to the invention with a commercially available particulate material, e.g. a hydrous kaolin which does not satisfy Equation 1 earlier, and by using the blend as an extender in a matt paint composition.
According to the present invention in a second aspect there is provided a paint composition which includes as an extender material a particulate mineral according to the first aspect optionally mixed with other particulate or white pigment material.
It is preferred that the paint composition is a matt paint composition. It is also preferred that the particulate mineral of the first aspect of the invention is a kaolin.
The composition may contain, in addition to the said extender material, ingredients which are usually employed in a matt paint composition. Thus, the composition may contain a latex binder and a primary pigment, namely TiO2. Other conventional additives such as one or more of dispersant(s), wetting agent, pH modifer, thickener(s), anti-foamer or defoamer and antifreeze coalescent may be incorporated in the composition. An example of a typical matt paint composition is given later.